1. Field of the Invention
The present invention relates to an apparatus and method for the shipping and storage of liquified gases. More particularly, the present invention relates to an apparatus and method for the shipping and storage of liquified gases of high value, such as, for example, liquid argon.
2. Description of the Prior Art
Argon is widely used in the manufacture of computer chips and in numerous other industrial and manufacturing processes where an inert atmosphere is needed. Liquified argon is obtained by fractionation in the liquification of air. Because argon constitutes less than 1.5% by weight of air, liquified argon is quite expensive compared, for example, to liquified air itself or liquified nitrogen. Indeed, liquified nitrogen, on a per unit basis, is about 10% of the cost of liquified argon.
Because the cryogenic processes used to liquify air and ultimately obtain liquified argon are sophisticated and capital-intensive, users of even large amounts of liquified argon rarely have such cryogenic facilities available. Accordingly, the liquified argon is commonly shipped in insulated tank trucks or railroad tank cars from the cryogenic producing facility to the users. The shipping of liquified argon invariably leads to considerable product loss. Liquid argon has a very low heat capacity. Accordingly, any external heat absorbed by the liquified argon causes the latter to heat up quickly, expand, and vaporize. This expansion/vaporization process increases the pressure on the interior of the shipping vessel, e.g., tank truck, until argon gas is allowed to escape through a suitable pressure relief valve (vent). For this reason, tank cars and other shipping vessels leave a dead or vapor space volume of about 10% of the volume of the shipping vessel to accommodate the vaporization/expansion.
Because of its value, every effort is made to minimize losses of liquified argon by vaporization. This obviously requires shipping vessels with elaborate insulation to minimize heat transfer between ambient surroundings and the liquified argon. However, even with the best insulation methods presently in use, it is not uncommon, particularly when liquified argon is shipped in rail cars over substantial distances, for there to be an approximate 10% by weight loss of the liquified argon due to vaporization/expansion and subsequent venting.
At atmospheric pressure, liquified nitrogen boils at about -196.degree. C. whereas liquified argon boils at about -186.degree. C. It is known to take advantage of this boiling point differential between liquified argon and liquified nitrogen to minimize losses of liquified argon. See, for example, Japanese Patent Publication No. 298444, incorporated herein by reference for all purposes. It is also well known to vent storage vessels and the like containing a volatile, liquified product to maintain a desired temperature from cooling due to heat of vaporization of the liquified product.
It would be economically quite advantageous if there were available an apparatus and method that could be readily retrofitted into existing shipping vessels, i.e., tank trucks or railroad tank cars, which would substantially eliminate any loss of liquified argon.